Faulty PRGM / RUN switch on 29C

[David Hayden]

The prgm/run switch on my 29C no longer works. The switch moves okay but the calculator stays in prgm mode.

Hopefully it just needs to be cleaned. I haven't found anything specific about opening and working on a woodstock so I thought I better ask for advice here before breaking out the screwdriver.

Any advice?

Thanks,
Dave

[Joe Horn]

The same switch got flaky on my HP-25 (internally similar enough to the 29C) back in the day. Opening the calculator was easy, and cleaning the switch's contacts was also easy and totally effective. Give it a shot and tell us about it!

[Maximilian Hohmann]

Hello!

(03-19-2019 06:43 PM)David Hayden Wrote:  Any advice?

Disassembling the calculator is straightforward: Carefully remove the two rubber feet under the display (e.g. by wedging them out with a tiny flat screwdriver or a needle) and unscrew the two screws (small phillips screws) underneath them. The calculator then comes apart with very little force which is best applied by pushing against the contacts of the power plug.
The calculator itself consists of two main pieces, the PCB and the keyboard, which are plugged together with two rows of connectors. Just pull them apart to separate them.

As a first test I would clean the connectors and reassemble the calculator. Often the fault lies not in the keys or switches, but in a bad connection between the two parts. If this does not help, I would try applying some contact cleaner spray under the keys and dry it with a hairdryer or compressed air. If that doesn't help, the keyboard itself need to be taken apart, which unfortunately can not be done in a non-destructive manner due to it's "heatstaked" construction. The chances of getting it working again are 50/50 and in any case it will never be as good as before once it has been taken apart... For a real good descrption of how this is best done we all will have to wait for Geoffs magic book :-)

Regards
Max

NB: I am right now in the process of making one good HP-29C from two broken ones. One has faulty chips and the other one has a keyboard which was totally destroyed by the previous owner in an attempt to repair it. From the outside it looks like new but not one key is working any more. So don't take the keyboard apart unless there is no other option!

[rprosperi]

Dave,

After you carefully clean the switch slider and contacts (don't bend the metal spring contact) you should lubricate the contacts and slider before you reassemble the unit. There are many conductive lubricants made just for electrical switches, but an inexpensive one that is fairly easy to find is "Lubriplate DS-ES". One tube will last you a lifetime of repair and cleaning projects. Katie turned me on this years ago and it works perfectly every time.

Good luck!

[David Hayden]

Thanks everyone. I'll give it a try and report the results.
Dave

[Maximilian Hohmann]

Hello!

As I am currently working on two HP-29C's I took the opportunity to take some photos for illustration:

This is how the PCB with the electronic components (including the LED display) is conneced and mechanically joined with the faceplate which contains the keyboard contacts. The photo shows the calculator facing downwards with the power connector rods sticking out to the top. The connection between the PCBs is through two such rows of goldized contacts, one between the topmost row of keys and the switches and the other below the lowest row of keys.



This is after pulling out the contacts (soldered to the component PCB) from their holes (which are on the keyboard PCB). If one or more of these contacts are corroded, rows or columns of keys or a switch will not work any more. The same happens of course if a trace on either PCB has been "eaten away" by corrosion.



That's what an intact keyboard PCB looks like. The black dots are the "heatstakes" which hold the PCB in place.



After mechanically removing the heads of these heatstakes (e.g. with a small knife or drill) the PCB can be separated from the calculator case:



These are heatstakes with their heads removed close-up:



And this is the part of the PCB where the switches are located. That's acttually the back side of the bit of PCB shown in the previous image. If a switch does not work, chances are high that there is some corrosion or other non-conducting stuff in that area.



Finally, the contacts of the switches. These look large in the photograph but are actually very tiny and can easily be lost upon disassembly of the calculator... Again, corrosion of the metal or a worn gold layer can cause them to stop working. There is absolutely no replacement for these contacts apart from other woodstock calculators.



Good luck with your repair!
Max

[rprosperi]

Excellent photos Max, thanks for sharing them. There is no substitute for clears pics like these for folks new to Woodstock repairs to understand the unique connectors between the 2 boards.

How will you re-attach the PCB where you removed the heat stakes? Similar photos of that critical step will be very useful for folks attempting future Woodstock repairs.

Thanks again for sharing.

[Maximilian Hohmann]

Hello!

(03-22-2019 12:44 PM)rprosperi Wrote:  How will you re-attach the PCB where you removed the heat stakes?

That's the difficult bit! Someone posted pictures on this forum some time ago about how he used Epoxy glue to hold the keyboard together. This I think needs some experimenting because most types of glue will not stick to thermoplastic material very well. In any case one must be very careful not to spill any glue inside the keyboard otherwise it will either cover the PCB contacts or block some keys.

Therefore taking apart the keyboard should really be the very last resort. One should always try to clean the contacts from the outside first.

Regards
Max

[Leonid]

(03-22-2019 12:44 PM)rprosperi Wrote:  How will you re-attach the PCB where you removed the heat stakes?

For a similar problem a few years ago I used a small hand drill (like this) to burr hole in "heat stakes" (but not an electric drill to fully control the drilling process!).
Then I fixed the PCB with tiny eyeglass screws and plastic washers (like this).
As result is a reliable and detachable design

[mfleming]

Goes to Amazon, clicks "Add to Cart" ...

[BobVA]

(03-22-2019 12:44 PM)rprosperi Wrote:  How will you re-attach the PCB where you removed the heat stakes? Similar photos of that critical step will be very useful for folks attempting future Woodstock repairs.

FYI: here's a thread about an approach similar to Leonid's that worked for the 15CLE. (Worked fine for me, too)

[David Hayden]

I finally got some time to tackle this repair and thought I'd report back.

Many thanks for the instructions on disassembling the 29C. The only thing I'd add is that the calc and case are also held in place with a small clip at the bottom of the calc, just behind the "Hewlett-Packard 29C" badge. After removing the screws and pushing the down on the charger contacts to separate the calc from the case, you can use a finger nail at the bottom to push the calc up slightly towards the display to disengage the clip.

Cleaning the PRGM/RUN switch would require removing the heat stakes so I tried cleaning the switch by putting a little DeOxit on a piece of paper and gently sliding it back and forth under the keyboard PCB. Cleaning didn't work.

Next I got my multimeter and realized that the switch shorts out two easily accessible traces. Low and behold, the switch was working fine!

So I cleaned the little tuning-fork shaped connectors that Maximilian's photos show. Still no joy.

At this point I notices something odd. When I put it back together, if I pushed on the bottom of the calc, various segments of the display would come on and off. Last night, in my Turkey-induced Thanksgiving food coma, I realized that I was pushing right over one of the connectors. It also occurred to me that while I cleaned the front and back of the connectors, it was actually the skinny sides that made contact with the holes they slid into when the calculator was assembled. So I planned to clean the little connectors again, this time using deoxit on a string or thread so I could get the edges.

This morning I disassembled it to clean again. But as I looked closely, I noticed something else: the tines of the little tuning forks of the bottom connector were closer together than the tines of the top one. If they were too close together, they wouldn't contact the holes then slid into and that might produce exactly the symptoms I was seeing. Look at Maxamilian's second photo and imagine the smaller tine bent in a little towards the larger one.

To fix this, I used a very small flat-head screwdriver. I slid it between the tines of each connector and gently turned the screwdriver to widen the gap a little. That did the trick and the calculator is now working again. Yay!